Analysis of Internal Friction Peaks in High Purity Molybdenum by a Viscoelastic Procedure Independent of the Relaxation Strength

C. L. MATTEO; O. A. LAMBRI; G. I. ZELADA-LAMBRI; P. A. SORICHETTI; J. A. GARCÍA

Tula, Russia

Conferencia; IMPERFECTIONS INTERACTION AND ANELASTIC PHENOMENA IN SOLIDS, IIAPS XI; 2007

Tula State University

In previous works [1, 2], it was reported that in mechanically deformed high purity single-crystalline molybdenum after annealing over the temperature for the mobility of vacancies a damping peak develops. The intensity of the damping peak at around 840 – 1050K depended on the degree of plastic deformation at room temperature, but it was not affected by a bias stress. Moreover the peak temperature and activation energy of this relaxation process increased with the temperature of the previous annealing of the sample, and it was independent of the crystal orientation. In addition, once the damping peak appeared after annealing, the damping values were independent of the amplitude of oscillation. The single crystals used in this work were prepared from zone refined single-crystal rods of molybdenum in A.E.R.E., Harwell (RRR @ 8000). Samples with the  <110> and <149>  crystallographic tensile axis were used to favour deformation by multiple and single slip; respectively. The samples were annealed and then deformed in extension at a constant speed of 0.03 cm/min, followed by torsion at room temperature. The damping peak measured in high purity single crystalline molybdenum could be the result of the superposition of more than one elementary process. In fact, although the peak appears at temperatures around 840K, the physical mechanism controlling this relaxation could be the drag of vacancies by the dislocation line assisted by diffusion, which is in agreement with the activation energy, reported about 1.6 eV. In contrast, when the peak appears at higher temperatures (1050K) it could be controlled by vacancy creation and diffusion along the dislocation line, in agreement with the measured activation energy of 2.7-2.8 eV. Nevertheless, it is not easy to distinguish the contributions of the two processes as they appear to overlap in the observed temperature range. In this work we will present a procedure, involving linear viscoelastic analysis, to discriminate the two possible contributions of the observed damping peak. An interesting feature of the procedure is that, for low damping samples, it can efficiently resolve experimental peaks that result from the superposition of different processes independently of the ratio between their relaxation strengths. This allows us to confirm that two different relaxation processes appear, one around 840K, and the other one at about 1050K, which can be related to the diffusion and creation of vacancies, respectively.   1) O.A. Lambri, G.I Zelada-Lambri, L.M. Salvatierra, J.A. García and J.N. Lomer, Mat. Sci. and Eng. A, 370, (2004), 222. 2) G.I. Zelada-Lambri, O.A. Lambri  and  J.A. García, J. of Nucl. Mat., 353 (2006), 127.